House Majority PAC, a so-called super political action committee that recently helped Democrats hold former Rep. Gabrielle Giffords' Arizona seat, is investing $16 million, while the 2.1 million-member Service Employees International Union is spending $3.7 million, according to an official from the Democratic-leaning group who spelled out the planned spending. The official spoke on condition of anonymity to freely discuss the plans.

The ad reservation targets at least 47 competitive House races that include vulnerable Republican incumbents as well as Democratic lawmakers facing strong challenges.

Democrats need a net gain of 25 House seats to win back the control they lost two years ago.